1. Field of the Invention
The present invention relates to a fuel cell having a plurality of electrolyte electrode assemblies interposed between separators. Each of the electrolyte electrode assemblies includes an anode, and a cathode, and an electrolyte interposed between the anode and the cathode.
2. Description of the Related Art:
Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates), and the electrolyte electrode assembly and the separators make up a unit of fuel cell for generating electricity. A predetermined number of fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, an oxygen-containing gas or air is supplied to the cathode The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as hydrogen-containing gas or CO is supplied to the anode. Oxygen ions react with the hydrogen in the hydrogen-containing gas to produce H2O or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric current.
Generally, the solid oxide fuel cell is operated at a high temperature in the range from 800° C. to 1000° C. The solid oxide fuel cell utilizes the high temperature waste heat for internal reforming to produce the fuel gas, and generates electricity by spinning a gas turbine. The solid oxide fuel cell is attractive as it has the highest efficiency in generating electricity in comparison with other types of fuel cells, and receiving growing attention for potential use in vehicles in addition to the applications in combination with the gas turbine.
Stabilized zironia has a low ion conductivity. Therefore, the electrolyte membrane formed of stabilized zirconia needs to be thin so that oxygen ions move through the electrolyte membrane smoothly for improving the power generation performance. However, the electrolyte membrane of the stabilized zirconia can not be very thin for maintaining the sufficient mechanical strength. Therefore, it is difficult to produce a large electricity using the membrane of stabilized zirconia in the solid oxide fuel cell.
In an attempt to address the problem, Japanese Laid-Open Patent Publication No. 6-310164 (prior art 1) discloses a solid oxide fuel cell system. In the solid oxide fuel cell system, a plurality of unit cells each having a small surface area are provided on each of metallic separators, and a fuel gas supply hole and an oxygen-containing gas supply hole are formed centrally in each of the unit cells. The prior art 1 is directed to provide a fuel cell system having an improved reliability in which the total surface area of the cells on the separator is large, and the substrate is crack-free.
In the prior art 1, the cells are interposed between the thin separators, and the separators and the cells are stacked alternately to form a fuel cell stack. Therefore, the rigidity of the separators is low. Since a substantial space corresponding to the thickness of the cells is formed between outer circumferential regions of the separators, the outer circumferential regions of the separators may be deformed, and thus, gas leakage may occur. Therefore, the desired power generation performance may not be maintained.
Further, Japanese Laid-Open Patent Publication No. 4-26068 (prior art 2) discloses another type of fuel cell system. As shown in FIG. 17, in the fuel cell system, a unit cell 1 is interposed between a pair of separators 2. Each of the separators 2 includes a pair of metallic thin plates 3, 4. The plates 3, 4 are joined together with a marginal plate 5 interposed between the plates 3, 4. The marginal plate 5 shields the outer circumferential area between the plates 3, 4. A gas channel 7 is formed in an inner area 6 between the plates 3, 4. The plate 4 has small through holes 8 for supplying a fuel gas to the anode or an oxygen-containing gas to the cathode.
In the fuel cell system, the small holes 8 are formed on the plate 4 of the separator 2. The plate 4 having a flat surface is in contact with the unit cell 1 without any space between the plate 4 and the unit cell 1. Therefore, the pressure between the separator 1 and the unit cell 1 is uniform. Namely, the pressure is uniformly applied to the entire surface of the unit cell 1.
However, in the prior art 2, a gap 9 is formed between outer circumferential regions of the separators 2. Therefore, when the fuel cell stack is tightened by a bolt, for example, the pressure may not be uniformly applied to the entire surface of the separators 2. Thus, the fuel cell stack may be deformed undesirably. Consequently, the pressure is not applied to the entire surface of the unit cell 1. If the unit cell 1 is damaged, gas leakage may occur due to the sealing failure.